DESCRIPTION

The
getopt()
function parses the command-line arguments.
Its arguments
argc
and
argv
are the argument count and array as passed to the
main()
function on program invocation.
An element of argv that starts with '-'
(and is not exactly "-" or "--")
is an option element.
The characters of this element
(aside from the initial '-') are option characters.
If
getopt()
is called repeatedly, it returns successively each of the option characters
from each of the option elements.

The variable
optind
is the index of the next element to be processed in
argv.
The system initializes this value to 1.
The caller can reset it to 1 to restart scanning of the same
argv,
or when scanning a new argument vector.

If
getopt()
finds another option character, it returns that
character, updating the external variable optind and a static
variable nextchar so that the next call to
getopt()
can
resume the scan with the following option character or
argv-element.

If there are no more option characters,
getopt()
returns -1.
Then optind is the index in argv of the first
argv-element that is not an option.

optstring
is a string containing the legitimate option characters.
If such a
character is followed by a colon, the option requires an argument, so
getopt()
places a pointer to the following text in the same
argv-element, or the text of the following argv-element, in
optarg.
Two colons mean an option takes
an optional arg; if there is text in the current argv-element
(i.e., in the same word as the option name itself, for example, "-oarg"),
then it is returned in optarg, otherwise optarg is set to zero.
This is a GNU extension.
If
optstring
contains
W
followed by a semicolon, then
-W foo
is treated as the long option
--foo.
(The
-W
option is reserved by POSIX.2 for implementation extensions.)
This behavior is a GNU extension, not available with libraries before
glibc 2.

By default,
getopt()
permutes the contents of argv as it
scans, so that eventually all the nonoptions are at the end.
Two other modes are also implemented.
If the first character of
optstring is '+' or the environment variable
POSIXLY_CORRECT
is set, then option processing stops as soon as a nonoption argument is
encountered.
If the first character of optstring is '-', then
each nonoption argv-element is handled as if it were the argument of
an option with character code 1. (This is used by programs that were
written to expect options and other argv-elements in any order
and that care about the ordering of the two.)
The special argument "--" forces an end of option-scanning regardless
of the scanning mode.

While processing the option list,
getopt()
can detect two kinds of errors:
(1) an option character that was not specified in
optstring
and (2) a missing option argument
(i.e., an option at the end of the command line without an expected argument).
Such errors are handled and reported as follows:

*

By default,
getopt()
prints an error message on standard error,
places the erroneous option character in
optopt,
and returns '?' as the function result.

*

If the caller has set the global variable
opterr
to zero, then
getopt()
does not print an error message.
The caller can determine that there was an error by testing whether
the function return value is '?'.
(By default,
opterr
has a nonzero value.)

*

If the first character
(following any optional '+' or '-' described above)
of optstring
is a colon (':'), then
getopt()
likewise does not print an error message.
In addition, it returns ':' instead of '?' to
indicate a missing option argument.
This allows the caller to distinguish the two different types of errors.

getopt_long() and getopt_long_only()

The
getopt_long()
function works like
getopt()
except that it also accepts long options, started with two dashes.
(If the program accepts only long options, then
optstring
should be specified as an empty string (""), not NULL.)
Long option names may be abbreviated if the abbreviation is
unique or is an exact match for some defined option.
A long option
may take a parameter, of the form
--arg=param
or
--arg param.

longopts
is a pointer to the first element of an array of
struct option
declared in
<getopt.h>
as

struct option {
const char *name;
int has_arg;
int *flag;
int val;
};

The meanings of the different fields are:

name

is the name of the long option.

has_arg

is:
no_argument (or 0) if the option does not take an argument;
required_argument (or 1) if the option requires an argument; or
optional_argument (or 2) if the option takes an optional argument.

flag

specifies how results are returned for a long option.
If flag
is NULL, then
getopt_long()
returns val.
(For example, the calling program may set val to the equivalent short
option character.)
Otherwise,
getopt_long()
returns 0, and
flag points to a variable which is set to val if the
option is found, but left unchanged if the option is not found.

val

is the value to return, or to load into the variable pointed
to by flag.

The last element of the array has to be filled with zeros.

If longindex is not NULL, it
points to a variable which is set to the index of the long option relative to
longopts.

getopt_long_only()
is like
getopt_long(),
but '-' as well
as "--" can indicate a long option.
If an option that starts with '-'
(not "--") doesn't match a long option, but does match a short option,
it is parsed as a short option instead.

RETURN VALUE

If an option was successfully found, then
getopt()
returns the option character.
If all command-line options have been parsed, then
getopt()
returns -1.
If
getopt()
encounters an option character that was not in
optstring,
then '?' is returned.
If
getopt()
encounters an option with a missing argument,
then the return value depends on the first character in
optstring:
if it is ':', then ':' is returned; otherwise '?' is returned.

getopt_long()
and
getopt_long_only()
also return the option
character when a short option is recognized.
For a long option, they
return val if flag is NULL, and 0 otherwise.
Error and -1 returns are the same as for
getopt(),
plus '?' for an
ambiguous match or an extraneous parameter.

ENVIRONMENT

POSIXLY_CORRECT

If this is set, then option processing stops as soon as a nonoption
argument is encountered.

_<PID>_GNU_nonoption_argv_flags_

This variable was used by
bash(1)
2.0 to communicate to glibc which arguments are the results of
wildcard expansion and so should not be considered as options.
This behavior was removed in
bash(1)
version 2.01, but the support remains in glibc.

ATTRIBUTES

For an explanation of the terms used in this section, see
attributes(7).

Interface

Attribute

Value

getopt(),
getopt_long(),
getopt_long_only()

Thread safety

MT-Unsafe race:getopt env

CONFORMING TO

getopt():

POSIX.1-2001, POSIX.1-2008, and POSIX.2,
provided the environment variable
POSIXLY_CORRECT
is set.
Otherwise, the elements of argv aren't really
const,
because we permute them.
We pretend they're
const
in the prototype to be compatible with other systems.

The use of '+' and '-' in
optstring
is a GNU extension.

On some older implementations,
getopt()
was declared in
<stdio.h>.
SUSv1 permitted the declaration to appear in either
<unistd.h>
or
<stdio.h>.
POSIX.1-1996 marked the use of
<stdio.h>
for this purpose as LEGACY.
POSIX.1-2001 does not require the declaration to appear in
<stdio.h>.

getopt_long() and getopt_long_only():

These functions are GNU extensions.

NOTES

A program that scans multiple argument vectors,
or rescans the same vector more than once,
and wants to make use of GNU extensions such as '+'
and '-' at the start of
optstring,
or changes the value of
POSIXLY_CORRECT
between scans,
must reinitialize
getopt()
by resetting
optind
to 0, rather than the traditional value of 1.
(Resetting to 0 forces the invocation of an internal initialization
routine that rechecks
POSIXLY_CORRECT
and checks for GNU extensions in
optstring.)

EXAMPLE

getopt()

The following trivial example program uses
getopt()
to handle two program options:
-n,
with no associated value; and
-t val,
which expects an associated value.

SEE ALSO

COLOPHON

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